Hmm, why didn't I see this topic earlier? I have the answer to this...

First of all, you maxed your strength stat, right?

'Strength' is the primary stat used for throwing. 'Throwing accuracy' just determines how accurate the object will fall where you want it to, but strength controls how far you can throw an object of a certain weight, which I'm sure you'd agree is by far more important for throwing a weapon that doesn't rely too much on accuracy to do its work

See, you can throw really far when you've got high strength. However, the further you can throw, the apex of the object's path of movement will get higher, and may even go beyond the 4 (or 2) elevation levels available in the playing map (basically, what I'm trying to say is that the height of the playing field is too small to allow a really strong soldier to throw an object without hitting the invisible ceiling, and the game likes to abort such a command rather than execute it).

For some reason, soldiers sometimes tend to throw objects up high in order to drop an object at a nearby tile rather than just tossing it there like we'd normally expect them to. Worse still, if you're standing under a ceiling (like the wing of the Skyranger, just to name an example), your throwing abilities can be changed significantly.

Therefore, to fix your problem, set your strength back to 70, which is the game's natural limit (unless you've got a really old version without the caps). You might still find some trouble throwing because of hitting the invisible ceiling (most notable in small spaces or in if you're really trying to throw a standard grenade or alien grenade/sonic pulser as far as possible), but it won't be as bad as, say, 160 or 255. You'll be able to throw a grenade at least half the map, which is quite a long way.